Olefins are traditionally produced from petroleum feedstocks by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefin(s), such as ethylene and/or propylene, from a variety of hydrocarbon feedstocks. Ethylene and propylene are important commodity petrochemicals useful in a variety of processes for making plastics and other chemical compounds.
The petrochemical industry has known for some time that oxygenates, especially alcohols, are convertible into light olefin(s). There are numerous technologies available for producing oxygenates including fermentation or reaction of synthesis gas derived from natural gas, petroleum liquids or carbonaceous materials including coal, recycled plastics, municipal waste or any other organic material. Generally, the production of synthesis gas involves a combustion reaction of natural gas, mostly methane, and an oxygen source into hydrogen, carbon monoxide and/or carbon dioxide. Other known syngas production processes include conventional steam reforming, autothermal reforming, or a combination thereof.
The preferred process for converting an oxygenate, such as methanol, into one or more olefin(s), primarily ethylene and/or propylene, involves contacting the feedstock with a catalyst composition, typically containing a molecular sieve catalyst. The product stream from such a process is a complex mixture comprising the desired light olefins, unconverted oxygenates, by-product oxygenates, heavier hydrocarbons and large amounts of water. The separation and purification of this mixture to recover the light olefins and other valuable by-products is critical to the overall efficiency and cost effectiveness of the process. In particular, it is important that the purification scheme produces products that are substantially free of impurities, which could adversely effect downstream processing.
For example, certain oxygenate components present in the product from an oxygenate to olefin conversion (OTO) process, particularly aldehydes and ketones, may cause problems in olefin recovery operations and in derivative manufacturing processes that feed and react C4+ hydrocarbons. Various schemes have therefore been proposed for removing aldehydes and ketones from the olefinic and C4+ hydrocarbon components of oxygenate conversion effluent streams.
U.S. Pat. No. 6,303,841 and U.S. patent application Publication No. 2002/0007101, published Jan. 17, 2002, disclose a process for producing ethylene from oxygenates in which the oxygenate conversion effluent stream is compressed in a multi-stage compressor to a pressure of 1050 to 2860 kPa (150 to 400 psia), preferably 1750 to 2450 kPa (250 to 350 psia), washed with methanol and then water to remove unreacted oxygenates and then contacted with caustic to remove carbon dioxide. The carbon dioxide depleted stream is dried with a solid desiccant and passed to a deethanizer zone to provide a light hydrocarbon feedstream comprising hydrogen, methane, ethylene and ethane, and a deethanized stream comprising propylene, propane, and C4+ olefins. The light hydrocarbon stream is passed to a demethanizer zone operating at a temperature greater than −45° C. to provide a bottom stream comprising ethylene and ethane and an overhead stream comprising hydrogen, methane, and ethylene. The bottom stream is fed to a C2 splitter zone to produce the ethylene product stream and an ethane stream, whereas the overhead stream is fed to a pressure swing adsorption zone to remove hydrogen and methane and produce an ethylene-containing stream which is combined with the oxygenate conversion effluent stream.
U.S. Pat. Nos. 6,403,854 and 6,459,009 to Miller et al. disclose a process for converting oxygenate to light olefins in which the reactor effluent is quenched with an aqueous stream in a two-stage process to facilitate the separation of hydrocarbon gases from any entrained catalyst fines, as well as to remove water and any heavy by-products such as C6+ hydrocarbons. A portion of the waste water stream withdrawn from the bottom of the quench tower is recycled to the quench tower at a point above where the reactor effluent is introduced to the quench tower. The vapor product stream from the quench tower is compressed, passed to an adsorption zone for the selective removal of oxygenates and then passed to a caustic wash zone for removal of carbon dioxide. The resultant carbon dioxide free light olefin stream is passed to a dryer zone for the removal of water and passed to a conventional light olefin recovery zone.
U.S. patent application Publication No. 2003/0130555, published Jul. 10, 2003, discloses a process for separating oxygenated hydrocarbons from the olefin product of an oxygenate to conversion olefins reaction. The product is initially sent to a cooling unit, such as a quench tower, from which cooled olefin product is separated as an olefin vapor stream. The water containing bottoms stream can be recycled through a heat exchanger for cooling and/or removed from the cooling unit to a first separator, such as a distillation column, to provide an oxygenated hydrocarbon product of reduced water content and remaining water as a bottoms product. The olefin vapor stream is compressed to at least 30 psia (207 kPa), preferably 100 to 500 psia (689 to 3447 kPa), and directed to a second separator that provides an olefin vapor product and a liquid oxygenated hydrocarbon-containing stream. The liquid oxygenated hydrocarbon containing stream can then be combined with the water containing bottoms stream or directly added to the first separator to provide an oxygenated hydrocarbon product recovered from the first separator that is reduced in water content and can be used as fuel or co-feed for the oxygenate reaction process. Before or after the compression step, the olefin vapor can be washed with methanol and/or water at a temperature of 40 to 200° F. (4 to 93° C.), preferably 80 to 120° F. (27 to 49° C.).
In addition, U.S. patent application Ser. No. 10/871394, filed Jun. 18, 2004, discloses a process for producing olefins from the vaporous first effluent stream from an oxygenate to olefin conversion reaction, said vaporous first effluent stream comprising C2 and C3 olefins, C4 hydrocarbons, and C2 to C6 carbonyl compounds. In the process, the temperature and pressure of the vaporous first effluent stream are adjusted to produce a second effluent stream having a pressure ranging from about 100 psig to about 350 psig (790 to 2514 kPa) and a temperature ranging from about 70° F. to about 120° F. (21 to 49° C.), wherein the second effluent stream contains about 50 wt. % or more C4 hydrocarbons based upon the total weight of C4 hydrocarbons in the first effluent stream. The second effluent stream is then washed with an alcohol to remove carbonyl compounds and produce a third effluent stream, whereafter the third effluent stream is washed with water to provide a fourth effluent stream comprising the C2 and C3 olefins and about 1.0 wt. % or less of the C2 to C6 carbonyl compounds.
All of the above references are incorporated herein by reference in their entirety.
The unconverted and by-product oxygenates removed from the olefin-containing product streams in the above processes are valuable materials and are generally recycled backed to the OTO reactor for conversion to olefins. However, these oxygenate-containing streams also typically contain heavy (C5+) hydrocarbons, including aromatic compounds, that are considerably less reactive than the other components in the OTO feed and so, if not removed, can build up to unacceptable levels in the the reaction/purification system. Moreover, separation of heavy hydrocarbons from oxygenates, such as methanol, is difficult by conventional fractionation. For example, the normal boiling point of methanol is 140° F. (60° C.), whereas those of hexane and benzene are 156° F. (69° C.) and 176° F. (80° C.) respectively, and conventional fractional distillation would need to employ an expensive column having many trays with high reboiler and condenser duties to make any appreciable separation There is therefore a need for an improved method for separating heavy hydrocarbons from OTO effluent streams.